Jains don’t eat onion mainly because of ahimsā (non‑violence) and spiritual purity.

1. Onion is a root (underground) vegetable

- When you take an onion, you have to uproot the whole plant. - This kills the plant and also disturbs/kills many tiny living beings (micro‑organisms) in the soil around the roots. - Jain dharma teaches us to avoid maximum possible violence, so root vegetables like onion, garlic, potato, etc. are avoided.

1. Anantkāy & more living beings

- Traditional Jain view is that many roots have very many life‑forms living in or around them. - Eating such things means harming more jīvas (souls) in one act, so it is avoided by those who want to keep their karmic load low.

1. Effect on mind and passions

- Onion (along with garlic) is considered to excite passions and restlessness. - A Jain aims for a calm, pure, controlled mind for sādhanā, so foods that disturb inner peace are not recommended.

1. Digambar & Shvetāmbara

- Both traditions accept ahimsā as the main reason. - In practice, almost all strict Jains (both Digambar and Shvetāmbara) avoid onion as part of general no-root-vegetable rule, even if individual families may differ in strictness.

In simple words: Jains do not eat onion because it involves more violence (himsa) to plants and tiny lives, and it is believed to disturb mental and spiritual purity.